Hi, You might find free-game [1] to be quite original, and it may help for the audience that's you're talking about a game rather than category theoretic stuffs. I also second Dan's suggestion though. [1] http://hackage.haskell.org/package/free-game && https://github.com/fumieval/free-game/tree/master/examples for example code On Thu, Oct 3, 2013 at 8:01 AM, Andres Löh <andres@well-typed.com> wrote:

Hi everyone.

I'll follow Simon's lead, and ask a similar question with a similar motivation. I'm going to talk about free monads at the upcoming Haskell eXchange next Wednesday. I'll not limit myself to a particular library, and I'm open to related approaches (e.g. "operational") as well.

I'm also looking for as many compelling examples as possible. Like Simon, I don't want to know anything secret or anything that you wouldn't like me to include in my talk. Most useful are pointers to existing libraries using free monads that I might have missed (for example, because they're new or very specialized).

Thanks a lot for your help in advance.

Cheers, Andres

-- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

-- Alp Mestanogullari

We use the "Yielding IO"<http://comonad.com/reader/2011/free-monads-for-less-3/>construction for the FFI from our programming language Ermine. That sits atop a free monad. In "PHOAS for Free" <https://www.fpcomplete.com/user/edwardk/phoas> I show PHOAS is also just a free monad, when you look at it right, so much of the syntax manipulation in agda/coq can be viewed as just another application of a free monad. Moreover, bound<https://www.fpcomplete.com/user/edwardk/bound>uses something that is "almost free", which is how I handle the rest of my syntax trees. ;) I use similar free/operational monads for manipulating systems of equations for stochastic differential algebraic equations. An example of using it to model circuits: type Real = Signal Double type Resistance = Real type Inductance = Real type Capacitance = Real type Current = Real type Voltage = Real data Pin = Pin { __v :: Voltage, __i :: Current } makeLenses ''Pin instance Connector Pin where cap = Pin <$> cap <*> cap equate (Pin v1 i1) (Pin v2 i2) = do v1 := v2 i1 := i2 flop :: (Connector a, Connector b) => (a -> Model b) -> b -> Model a flop f b = do top <- cap b' <- f top equate b b' return top twoPin :: Pin -> Model (Pin, Voltage) twoPin p = do n <- cap p^._i + n^._i := 0 return (n, p^._v - n^._v) basic :: Pin -> (Voltage -> Model ()) -> Model Pin basic p k = do (n,u) <- twoPin p k u return n resistor :: Resistance -> Pin -> Model Pin resistor r p = basic p $ \u -> r * p^._i := u inductor :: Inductance -> Pin -> Model Pin inductor l p = basic p $ \u -> l * der (p^._i) := u capacitor :: Capacitance -> Pin -> Model Pin capacitor c p = basic p $ \u -> c * der u := p^._i conductor :: Conductance -> Pin -> Model Pin conductor g p = basic p $ \u -> p^._i := g * p^._v -- | @transformer l1 m l2@ represents a transformer with -- primary inductance @l1@, coupling inductance @m@, and secondary inductance @l2@ transformer :: Inductance -> Inductance -> Inductance -> Pin -> Model Pin transformer l1 m l2 p@(Pin v1 i1) = do (n@(Pin v2 i2),u) <- twoPin v1 := l1 * der i1 + m * der i2 v2 := m * der i1 + l2 * der i2 return n Then I can fold together circuits with things like: circuit = do p <- cap cn <- capacitor 0.00047 =<< resistor 1000 p ind <- inductor 0.01 =<< resistor 2200 p acn <- acVoltageSource 12 p gn <- ground cup [cn,ind,acn,gn] or I can model stocks: type Real = Signal Double type Rate = Real data Stock = Stock { _price, _drift, _volatility :: Real } makeLenses ''Stock instance Connector Stock where cap = Stock <$> cap <*> cap <*> cap equate (Stock p d v) (Stock p d2 v2) = do p1 := d1 d1 := d2 v1 := v2 stock :: Model Stock stock = do model@(Stock s mu sigma) <- cap w <- brownianMotion der s := mu * s + sigma * der w assume (<=) s 0 return model -- @forward t r s@ calculate the forward price of a stock @s@ at time @t@assuming a risk free rate @r@ forward :: Time -> Rate -> Stock -> Model Price forward end rate stock = do t <- now assume (<=) t end return $ stock^.price * exp (rate * (end - now)) I also use the free monad as part of a variant on Tim Sheard's 2-level unifier. If you look Wren Thornton's version of unification-fd<http://hackage.haskell.org/package/unification-fd-0.8.0/docs/Control-Unification.html>the UTerm type is a free monad. My machines package uses a CPS'd free monad (Plan) to build up an explicit fixed point (Machine). wl-pprint-extras uses a free-monad based Doc to permit me to sprinkle annotations about color or whatever I want into the document. That's most of what I can come up with off the top of my head. -Edward On Thu, Oct 3, 2013 at 2:01 AM, Andres Löh <andres@well-typed.com> wrote:

Hi everyone.

I'll follow Simon's lead, and ask a similar question with a similar motivation. I'm going to talk about free monads at the upcoming Haskell eXchange next Wednesday. I'll not limit myself to a particular library, and I'm open to related approaches (e.g. "operational") as well.

I'm also looking for as many compelling examples as possible. Like Simon, I don't want to know anything secret or anything that you wouldn't like me to include in my talk. Most useful are pointers to existing libraries using free monads that I might have missed (for example, because they're new or very specialized).

Thanks a lot for your help in advance.

Cheers, Andres

-- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com _______________________________________________ Libraries mailing list Libraries@haskell.org http://www.haskell.org/mailman/listinfo/libraries

Andres Löh <andres <at> well-typed.com> writes:

Hi everyone.

I'll follow Simon's lead, and ask a similar question with a similar motivation. I'm going to talk about free monads at the upcoming Haskell eXchange next Wednesday. I'll not limit myself to a particular library, and I'm open to related approaches (e.g. "operational") as well.

I'm also looking for as many compelling examples as possible. Like Simon, I don't want to know anything secret or anything that you wouldn't like me to include in my talk. Most useful are pointers to existing libraries using free monads that I might have missed (for example, because they're new or very specialized).

Thanks a lot for your help in advance.

Cheers, Andres

I my "pianola" package, I use a free monad (in module Pianola.Protocol) to abstract away the connection details of RPC calls, along with the reading/writing from the handles. I use another free monad (in module Pianola.Pianola, but not exported) to sequence observational interactions with an external object accessed through the network. The functor is the Kleisli arrow of a monad. The observations can return no results, or more than one result. Depending on the interpreter, more than one result can be treated as an error, or at least merit a warning. The current interpreter just chooses the first result and keeps going. Cheers. http://hackage.haskell.org/package/pianola-0.1.1 http://hackage.haskell.org/package/pianola-0.1.1/docs/Pianola-Protocol.html http://hackage.haskell.org/package/pianola-0.1.1/docs/src/Pianola- Pianola.html

Andres Löh wrote:

Hi everyone.

I'll follow Simon's lead, and ask a similar question with a similar motivation. I'm going to talk about free monads at the upcoming Haskell eXchange next Wednesday. I'll not limit myself to a particular library, and I'm open to related approaches (e.g. "operational") as well.

I'm also looking for as many compelling examples as possible. Like Simon, I don't want to know anything secret or anything that you wouldn't like me to include in my talk. Most useful are pointers to existing libraries using free monads that I might have missed (for example, because they're new or very specialized).

I have included a couple of examples with the operational package: <https://github.com/HeinrichApfelmus/operational/tree/master/doc/examples#readme> My personal highlights are: BreadthFirstParsing.hs An implementation of parser combinators that does not have the space leak associated to the depth-first approach s -> [(a,s)] . Instead, parsers are evaluated in a breadth-first fashion, so that the input can be consumed in an on-line fashion. This is equivalent to Koen Classen's "parallel parsing processes", which I believe are used to implement the 'Text.ReadP' module. WebSessionState.lhs A program is written as a sequence of instructions, in this case a web session ("shopping cart checkout"). However, unlike the code may suggest, the program actually exits before completing and continues at the last instruction when started. No persistent state is kept in RAM. I learned this idea from Peter Thiemann's WASH/CGI library. TicTacToe.hs Similar to the previous example, but dialed up a notch. A game is implemented using a monad that returns player input. Internally, however, the player input can come from many sources: user, computer, or even replays, depending on how the free monad is interpreted. A less useful but still interesting example is `State.hs`, which implements the state monad using only s -> a instead of s -> (a,s) . Another example that I forgot to add when writing the package, but which also one of my highlights, is the probability monad. Using operational, it is possible to use the same code for both calculating complete probability distributions and sampling from said probability distributions. Yet another example for free monads/operational is the sunroof package <http://hackage.haskell.org/package/sunroof-compiler>, which is an EDSL that compiles to JavaScript. JavaScript statements are sequenced in a monad, but the monad is then interpreted as a syntax tree ("deep embedding"). Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com

Hi again. I'd like to thank everyone who replied to my mail regarding examples for the use of free monads. I ended up using very few of the suggestions, due to time constraints. If you want to look at the final result, it's available here: http://skillsmatter.com/podcast/home/monads-for-free I nevertheless gained a lot of insight by the examples provided. They were therefore very helpful to me, also in the final stages of the preparation of my talk. I feel motivated to produce a longer version of the talk or a follow-up at some point, showing more examples. Thanks again! Cheers, Andres -- Andres Löh, Haskell Consultant Well-Typed LLP, http://www.well-typed.com